Iranian security forces yesterday threatened a "decisive and revolutionary confrontation" with opposition demonstrators if protests continued against Mahmoud Ahmadinejad, who claimed victory in the country's presidential elections.

The threat from the Revolutionary Guards signalled that the powerful conservative force could intervene directly to end the political crisis. Protests continued yesterday despite several deaths over the weekend and increasingly violent methods used by the police and the pro-regime Basij paramilitaries.

As Iranian officials and the state media stepped up their allegations of British involvement in the protest movement, the British embassy in Tehran began evacuating the families of members of staff, and the Foreign Office advised against non-essential travel to Iran.

The parliamentary speaker, Ali Larijani, called for a "revision" of diplomatic ties with the UK in the light of what he alleged was British meddling, an allegation denied on Sunday by the foreign secretary, David Miliband.

British diplomats warned, however, that a further worsening of the bilateral relationship was likely in the next few days in view of Tehran's allegations.

The pro-government Fars news agency quoted a former member of a conservative Islamic student movement as saying: "If Britain continues its blatant intervention and malevolence, Iranian students will close down the British house of spies in Iran, like they did the US house of spies."

The European Union, represented by the Czech government, summoned Iran's chargé d'affaires and "categorically rejected" all allegations of interference, and the Italian embassy said it was available to offer humanitarian aid to injured demonstrators.

At least 17 protesters have been killed so far, according to official figures. A video showing the death of a young female demonstrator, Neda Agha Soltan, who was shot dead by a sniper on Saturday, was widely circulated in Iran and around the world.

The image has helped to instil fear in opposition ranks but some observers predicted that it might also galvanise the wavering protest movement in the weeks ahead as it mourns her death.

Following the bloodshed on Saturday, the turnout for demonstrations yesterday was much smaller than last week. Only a few hundred protesters gathered in Tehran's vast Haft-e-Tir Square and they appeared to be outnumbered by riot police firing teargas and Basij militiamen on motorbikes armed with rifles, as helicopters flew overhead.

Gunmen were visible at windows of government buildings around the square, as well as policemen with cameras taking pictures of the demonstrators. The judiciary has threatened to set up special courts to try dissidents.

The government backlash against the protesters intensified as the regime acknowledged serious problems with the 12 June elections. The guardian council, a group of 12 conservative clerics tasked with investigating the vote, said that more votes had been cast in 50 electoral districts than there were registered voters. However, a council spokesman insisted that such "discrepancies" would not overturn the president's election.

Alongside the riot police, the government's principal weapon against the demonstrators has been the Basij militia, a paramilitary subsidiary of the Revolutionary Guard.

Yesterday the 125,000-strong Revolutionary Guard corps threatened to intervene directly to quash the protests. A statement on its website said the demonstrators should "be ready for the decisive and revolutionary confrontation with the IGRC, Basij and other security forces".

The intelligence department of the police issued a statement of its own urging Iranians to help police officers identify "the main elements who have been behind the recent riots", while a member of the judiciary said special courts should be set up to try them.

In an attempt to stifle protests before they began yesterday, Haft-e-Tir Square was ringed by police and Basij, who were stopping pedestrians to question them and scrutinise their identity papers. Any groups of more than three people were stopped on suspicion of joining an illegal protest.

Police could be seen filming from the surrounding rooftops and from cars and motorbikes, in a apparent move to identify protest leaders. Police armed with rifles could be seen at the windows of government buildings around the square.

However, demonstrators succeeded in gathering on the north side of the square where they chanted Allahu Akbar (God is Great) before the police fired teargas. As the crowd scattered they were sheltered by locals in their homes and offices in a gesture of solidarity.

"I'm was badly beaten by the riot police on Saturday," said Mostafa, a 25-year-old university graduate. "I can still feel the pain but the pain doesn't matter – what matters for me is that they got to my spirit. I'm here in Haft-e-Tir Square to take my spirit back from this government. To get back my vote and the blood of whole innocent people who were killed by this government."

The police also put on a significant show of force around the university, stopping and questioning students. A young man was pushed to the ground and hit with batons as he tried to escape. A young woman was also attacked but her cries for help were ignored by the crowd, which appeared depressed and helpless.

Leili, a 32-year-old artist, said: "They were thinking that they could stop people claiming their rights after their brutal attack and killing people on Saturday but they are wrong, people will not easily pull back.

"We are still here although we don't have anything, not even a newspaper to cover what's happening, and they have rifles and everything. This is a big 'no' to the supreme leader."